Heretofore, single passband synchronizers 100 (see FIG. 1A) have employed voltage controlled oscillators (VCO) 110 in conjunction with phase detectors 120 and filters 130 to form a phase locked loop (PLL). The output clock F:out is synchronized with the input data rate F:in within the passband for providing clock pulses to the data stream from memory. Floppy disc 150 (see FIG. 1B) has a series of concentric circular recording tracks divided into record sectors 154. Each record sector 154 includes an identification word 158 (32 bits) followed by a data word 162 (128 bits). Each data word 162 may have a recording density and subsequent data rate F:in slightly different from the recording density of the associated identification word 158 (due primarily to variations in drive speed between the data systems employed for recording, or the drive speed variations within a single system at different times). Data rate synchronizer 100 must independently resynchronize to the data rate of each new word 158 and 162.
An acquisition code 158c and 162c is provided just before each identification word 158 and data word 162 for permitting synchronizer 100 to adjust to each new data rate prior to actually receiving the bits thereof. Each acquisition code is recorded with a predetermined sequence of "1"s and "0"s at the same time as the subsequent word, and therefore necessarily at the same data density as the subsequent word. The acquisition code length defines the acquisition period within which the PLL of synchronizer 100 must adjust to the small variations inherent in each new F:in.
The PLL frequency response parameters, loop frequency F1 and damping factor Df, are determined primarily by filter 130. The response must be optimized, fast enough to track the new data rate within the associated loop acquisition period, but not so fast as to ring extensively or oscillate.
The conventional passband center frequencies are 125,000 Hz for single density 5 inch discs, 250,000 Hz for double density 5 inch discs and single density 8 inch discs, and 500,000 Hz for double density 8 inch discs. Previous attempts to obtain multiband operation compatible with all of these conventional passbands have involved modifying the circuit components of the prior art VCO 110. These attempts have had limited success because the circuit components responsible for the operational frequency F:out also affect the loop response parameters; and therefore create a different loop response time and damping factor for each passband.
The interdependence of the VCO circuit and PLL are illustrated below for the two basic types of loop filters 130 shown in FIG. 1C and 1D.
______________________________________ PASSIVE FILTER ACTIVE FILTER ______________________________________ ##STR1## ##STR2## Df = Df = ##STR3## ##STR4## ______________________________________
where
Lf is the loop frequency of PLL,
Df is the damping factor PLL,
T1 and T2 are the response times of filter 130 ("PHASELOCK TECHNIQUES" p 7-9 by Gardner, Wiley)
Kpd is the sensitivity of phase detector 120,
Kosc is the sensitivity of VCO 110, and
Kosc=dF:out/dVcn=K/CrVr
Adjusting the operational frequency F:out by altering the VCO circuit parameters generates a corresponding change in the loop acquisition time.